tableview load image async but sometimes some cell will never load successfully? - objective-c

Question

I want tableview to load images async,
- (NSView *)tableView:(NSTableView *)tableView viewForTableColumn:(NSTableColumn *)tableColumn row:(NSInteger)row {
StickerListItemView *rowView = [tableView makeViewWithIdentifier:StickerTableViewIdentifier
owner:self];
if (!rowView) {
rowView = [[StickerListItemView alloc] initWithFrame:NSMakeRect(0, 0, 460, 300)];
rowView.identifier = StickerTableViewIdentifier;
id obj = [self.stickerListArray objectAtIndex:row];
if ([obj isKindOfClass:[StickerListModel class]]) {
StickerListModel *listModel = (StickerListModel *)obj;
rowView.productId = listModel.productId;
rowView.title = listModel.name;
rowView.intro = listModel.intro;
rowView.status = [self hasStickersDownloaded:listModel.productId];
[rowView setDelegate:self];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^(void) {
NSImage *image = [self getPreviewImageWithMd5:listModel.md5OfPrieviewImage];
//****************************************
NSLog(#"get image at index %ld",(long)row);
if (image) {
dispatch_async(dispatch_get_main_queue(), ^{
rowView.image = image;
[rowView setNeedsDisplay:YES];
//**************************************
NSLog(#"set image at row %ld",(long)row);
});
}
});
}
}
return rowView;
}
Sometimes, it will load successfully, but sometimes one cell will never load the image, just like the picture below: the
I wait for long time, but the third image never loaded.

Answer 1

It looks like this line is returning nil.
NSImage *image = [self getPreviewImageWithMd5:listModel.md5OfPrieviewImage];
You should check the value of image to make sure it's not nil and possibly add some error handling. Maybe something like this.
if (image) {
dispatch_async(dispatch_get_main_queue(), ^{
rowView.image = image;
[rowView setNeedsDisplay:YES];
NSLog(#"set image at row %ld",(long)row);
});
} else {
NSLog(#"Received nil image for row %ld", (long)row);
//possibly set a placeholder image for this row instead
}